Bottom Skins Fitting

Bellcrank Access Plate in Outboard Skin

The bellcrank acces plate for the outboard bottom skin is shown on drawing 21. The plate is held on with screws and is made to sit flush with the bottom skin. This is usually done by making a reinforcing ring out of 0.040 material which is riveted to the inside of the skin. The inside opening in the reinforcing ring is smaller than the access plate, and platenuts are put in it for the access plate holddown screws to go into. Some builders joggle the edge of the skin instead (cut the opening smaller than the access plate and put a double bend where the edge of the access plate hits so that part of the skin is .040 inches offset from the rest of the skin. With the right tools this can be quicker but the plans method will result in a smaller gap.

The predrilled wingskins come with this already done, so you only have to make the access cover. Follow the special directions that come with the predrilled skins, and skip the rest of this section.

There are two problems with the present plans and kit; the part of the reinforcing ring that holds the nutplates for the access plate is a little too narrow, and if the access plate is made to the dimensions explicitly shown on the plans the piece of material they are to be cut out of may be too small (the material provided to cut out two plates is 13" wide so each plate must be 6 1/2" or less);. Compensations are described below. In addition, the nutplates will hit the rib reinforcing angle and the flange of the bellcrank support gusset. Modifications for this have already been done in previous sections.

1. Place the paper on the bottom side of the wing over the area where the aileron bellcrank supports are, with one edge of the paper along the trailing edge of the leading edge skin and another edge so it just covers the flange of the #8 rib (the one that is next inboard from the rib that holds the aileron bellcrank supports. Mark on it where the centerline of the bellcrank rib lies. Mark the location of the main spar flange trailing edge. Remove the paper. Draw the features of the reinforcing ring and bottom skin cutout on it (from drawing 21 or 19A, but with slight modifications) as follows:
2. Draw the rearward inside edge of the reinforcing ring slightly under 6" back from the leading edge of the template (which corresponds to the edge of the leading edge skin, not the edge of the spar flange) instead of 6 1/8 as the plans imply and the sides of the inside edges 8" apart (instead of 8 1/4" as the plans indicate, to make the reinforcing ring larger for the nutplates) and equidistant from the rib centerline.
3. Draw the rearward inside edge of the cover plate slightly under 6 1/2" back from the leading edge of the template and the sides of the plate 9" apart and equidistant from the rib centerline.
4. Draw the outside edges of the reinforcing ring as shown in the plans.
5. Draw the radiuses to match, but use 1" instead of 1 1/8" for the inner edge of the reinforcing ring. Position the centerpoint for the radiuses (radii to the educated) based on where your edge lines are rather than using the 4 1/2" dimension on the plans. Make sure the radius center points are clearly marked.
6. Carefully cut out the inside-most part of the template and set aside (this leaves a "D" -shaped hole, not "C" -shaped hole in the template).
7. Set the template on the wing to insure that the opening is in the right place; the spar should just be covered at the opening, the opening should be centered on the bellcrank rib, and the end of the cutout of the rib flange should be visible.
8. Mark the outline of the cutout piece on the 0.040 material for the reinforcing ring; this is the inside edge of the ring (do both rings now). Also mark the radius centers, then mark the outside edges of the reinforcing rings. Cut out the reinforcing rings and deburr the edges.
9. Cut out the next set of lines on the template, leaving a cutout that represents the access plate outline. Use the template to mark and cutout the access plate opening in the bottom skin and the access plate itself from the material for that. The bottom skin opening should be made a little undersized; then trim it to fit the access plate with only a very small gap. Mark "inside" on the inside surface of the access plate.

Some builders mount a power supply for a strobe light at each wing tip. A separate supply for each wing is used rather than a single supply in the fuselage because the large current and voltage transient on the wire from the supply to the light can generate considerable electrical noise. The supply is mounted on the inside of the tip rib if the builder plans to put antennas in the tip or to permanently attach the tip. In this case access to the strobe light power supply is through another access plate in the bottom skin. This access plate should be made in the same way as the bellcrank access plate.

1. Mark centerlines on the main rib flanges; extend these lines onto the leading edge and tank skins. If you don't have a hole transfer jig (strap duplicator) extend them around to the trailing edge of the rear spar.
2. Clamp the outboard bottom skin in place on the wing with the outboard edge aligned with the outboard edge of the leading edge skin. Mark the location of the inboard edge of the skin on the rear spar.
3. Clamp the bellcrank access hole reinforcing ring in its proper position on the inside of the bottom skin. Drill the skin to it #40 (or #41). Mark positions for the access plate to reinforcing ring screws on the skin next to the reinforcing ring, making sure the nutplates to be put in later will not hit the bellcrank support gusset or the rib reinforcing angle. Secure the access cover in the opening and drill the screw holes #19 or 11/64". If you have the predrilled wing skins, you only need to drill the access cover screw holes through the cover and the bottom skin.
4. Remove the cover plate and ring, marking the outside surface of each.
5. Mark the cutout for the aileron area on the outboard bottom skin as shown in SK-51 in the construction manual; the trailing edge of the skin in that area should line up with the start of the radius of the spar as shown in drawing 16. The cutout goes in from the outboard edge just far enough to clear the aileron hinge bracket. This is already done for you if you have the predrilled wing skins.
6. Remove the skin and make the aileron clearance cutout.
7. If you have a strap duplicator drill a #41 hole in the rear spar flange at each rib centerline; if not, make a rib centerline position template for the rear spar area using the skin end mark at the inner rib as a reference point.
8. Put the skin back on and note whether you need to trim the leading edge to eliminate any gaps. If so, position the skin so that it butts against the leading edge skin or overlaps it, but no gap exists. Then loosen the skin at the top and shave it until it butts all along the leading edge skin with no gaps or overlaps.
9. Make a rib rivet position template using the leading edge skin trailing edge as a reference point. Mark the starting point for the rivets for the short aileron support bracket rib, the aileron bellcrank rib (#73.5), and the bellcrank support gusset on it.
10. Clamp the skin back in position and mark the rib rivet positions. Locate the rear spar-rib centerline points using the strap duplicator or template. Do not mark for rivets at the innermost rib.
11. Mark the rivet line along the main spar 1/4" up from the trailing edge of the spar. Mark the rivet line for the rear spar along its centerline. Mark the positions of the rivets along the main spar 7/8" or less apart and corresponding to the leading edge skin rivets. Mark the positions of the rivets for the rear spar 1" or less apart, with one at the centerline of every rib. Do not mark for rivets at the innermost rib.
12. Drill these rivet holes #41 or 3/32".
13. Find good positions for four screws to hold the bellcrank access plate to the spar; put marks on the leading edge skin to indicate them. Attach the access plate to the skin and drill the screw holes #19 or 11/64". Remove the access plate. Drill the spar for countersunk platenuts, dimple the screw holes in both the cover and the spar, and mount the platenuts. Drill and countersink the reinforcing ring for platenuts, dimple the large holes of both the ring and the access plate for #8 screws, and mount platenuts on the inside surface of the ring.

1. Clamp the inboard bottom skin to the wing butted up against the tank skin and overhanging the centerline of the outermost rib (where the outboard skin starts) by at least 1 3/16".
2. Mark all the rib rivet locations for 1 1/4" spacing including the outboard edge row (see SK-50), then erase every other one at the root rib starting with the first one (this is for the fuselage skin-to-wing attach screws).
3. Drill the marked holes #41 or 3/32" (#40 if you plan to countersink this skin).
4. Mark the rivet line along the main spar 1/4" up from the trailing edge of the spar. Mark the rivet line for the rear spar along its centerline. Mark the positions of the rivets along the main spar 7/8" less apart with one on the centerline of every rib. Mark the positions of the rivets for the rear spar 1" or less apart with one at the centerline of every rib.
5. Drill these rivet holes #41 or 3/32" (#40 if you plan to countersink this skin).

1. Remove and debur both bottom skins.
2. Bevel the outside of the outboard bottom skin at the innermost rib as shown in drawing 21 so there will not be an abrupt edge where the inboard skin fits over it. Bevel the inside and outside of the inboard skin at the same location; do not overdo it to where you have a sharp, weak edge. Do this just at the leading edge; doing it all along the skin to the trailing edge would look better but would weaken the structure. Just round off the edge of the outboard skin the rest of the way back.
3. Dimple the skins. The Inboard skin can be countersunk if desired.
4. Prime the areas of the skins and reinforcing ring where they will be joined, and preferably all of the inside of the skins.
5. Rivet the reinforcing ring to the outboard skin.

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